The invention relates to a movable projector unit for welding control purposes with pipe-lines and the like. A projector capable of emitting gamma rays can be used to control the homogeneity of weldings in or with pipe-lines with the aid of gamma radiography. A radio-active source of gamma rays is used in this case, which source must be positioned in or outside the pipe-line at the location of a welded joint. If the radio-active source is not employed it is positioned in a protective housing, having, for example, uranium as shielding material. If an exposure is to be taken, the radio-active source is moved by a movement mechanism to an exposure position outside the protective housing, after which, in this position and during a certain exposure time, an image of the weld is fixed on a rontgen film.
In order to control the welds rapidly from location to projector is joined with a motor driven carriage or crawler in accordance with the invention under application of an electronic control unit. In case such a carriage is employed in pipe-lines of different cross-section, said carriage can have different dimensions as well. For example for pipe-lines having a cross-section from 15 cm to 150 cm, different carriages are used having succeeding larger dimensions.
Said carriages of different dimensions, however, all have a similar mechanism for moving said radio-active source from the protective housing to the exposure position outside said housing. The radio-active source, depending upon its type, must be displaced over a distance of from 50 mm to 70 mm in a suitable fitting channel in order to avoid leakage radiation through slits.
From the state of the art a movement known mechanism is provided with a solenoid for obtaining a direct linear movement. As the stroke length of a common solenoid is only about 30 mm, this stroke length is mechanically extended to the desired value. Add to this the fact that the current consumption of the solenoid is relatively high and even becomes higher through wear, dust or corrosion, by which the movement is somewhat dragged. Apart from the hold current one has to reckon also with a high starting current. The adjustment of the solenoid having a mechanical path extension and a draw-spring is critical and in case of slight dirtying by dust and other impurities brings about problems both in forward and in backward direction. Also said solenoid has to stay in working condition both at high and at low temperatures in or near pipe-lines, for example, in tropical and iced regions. When a solenoid burns through combination of high temperature and wear then a withdrawal of the radio-active source into the protective housing is unsure even after cooling off to normal surrounding temperature. The deformation and insulative lake will hamper the armature in the solenoid in its movement.
In view of above and as the increase of the stroke length in a solenoid is unsure, one has switched from the direct linear movement by means of a solenoid to an indirect linear movement. A rotating movement then is converted into a linear movement with the aid of a small motor having a toothed wheel and a gear rack, to which the radio-active source is coupled. At the end of the stroke said gear rack is held by a catch operated by a small solenoid. The required rotation time of the motor is stored in an electronic circuit. The period during which said small solenoid is excited, is given by the required exposure time. After the exposure the magnetic coupling drops off by which the return draw-spring urges the gear rack to the safe rest position. As the toothed wheel of the motor is provided with a free running wheel, said radio-active source with the aid of the return spring is moved back from its exposure position to the safe rest position as soon as the catch drops. Also, this system in practice entailed a number of difficulties resulting from the complexity and the large number of fine mechanical components, which components do not always function safely.